1. Field of the Invention
The present invention relates to the photopolymerization of hydrophilic monomers and to the water soluble polymers resulting therefrom, as well as to the use of such polymers as polymeric flocculants.
2. Description of the Prior Art
It is well known to this art to prepare polymeric flocculating agents via photopolymerization of acrylic monomers, specifically acrylamide, methacrylamide, acrylic or methacrylic acids, optionally the salts thereof, and quaternary ammonium salts derived from dialkylaminoalkyl (meth)acrylates.
A plurality of characteristics and properties is typically required of such organic polymeric flocculating agents and of the respective processes for the preparation thereof, certain of which are often contradictory and mutually inconsistent, i.e.,: the polymeric flocculating agents must be water soluble; they must have molecular weights as high as possible (high intrinsic viscosity); when dissolved in water, they must, if possible, leave no insoluble residue; their speed of dissolution in water must be as high as possible; it must be possible to produce such polymeric flocculants by polymerization of an aqueous monomer solution which is as highly concentrated as possible in order that the output of the production facility will be high, while, simultaneously, the heat generated during polymerization, or exotherm, must be quickly dissipated such that as low a temperature as possible is maintained in the polymerization recipe to prevent degradation of polymer, a reduction in molecular weight, and the possible formation of insoluble fractions in the water; and, finally, it is necessary that the polymerization quickly proceed to as advanced a stage as is possible, in order that the content of residual monomer or monomers in the flocculant be as low as possible. Further, if it is necessary to include photopolymerization additives, same should be, to the extent possible, both odorless and nontoxic. Finally, it is desirable that the pH of the solution subjected to photopolymerization not be too high, especially when the monomer or monomers utilized are susceptible to saponification.
It is moreover, quite difficult to attain and reconcile all of the foregoing objectives when the monomers are pure, it is even more difficult when the monomers contain impurities.
It of course is a known fact that acrylamide contains impurities, particularly that acrylamide resulting from the hydrolysis of acrylonitrile and especially the sulfuric acid hydrolysis thereof. These impurities, as of yet unknown in nature, normally are acidic in character and this permits the evaluation of the degree of impurity of the monomer by measuring its acid number. It is, however, not enough to simply neutralize the monomer and thus to neutralize the impurities to eliminate the deleterious effects thereof, because acid number is not an absolute value reflecting true purity of the acrylamide.
The acid number is determined by neutralization with N/10 potassium hydroxide [color change indicated by phenol red] of a solution containing 90 cm.sup.3 pure water and 10 g acrylamide, and it is expressed in milligrams of KOH necessary to neutralize 1 g acrylamide.
An increase in the acid number of acrylamide [acrylamide originating from the sulfuric acid hydrolysis of acrylonitrile and without any special neutralization] is accompanied by enhancement of the deleterious effects attendant use of such monomer. These deleterious effects tend to manifest themselves initially by prolonging that amount of time required for dissolution of the resultant polymer in water, but may even give rise to the formation of insoluble gels. With an acid number of 0.4, appreciable deleterious effects are already observed; the time period required for dissolution of the flocculant in water may be multiplied by a factor of 5 or even 10 simply as a result of an increase in the acid number of but a few tenths of one.
The aforesaid deleterious effects manifest themselves not only for the abovementioned impurities of acrylamide, but also for all types of other impurities, of other monomers. Thus, acrylic acid often contains acrolein which gives rise to those deleterious effects when present in but very small amounts; its influence is observed even at 5 ppm [parts per million] of acrolein in the acrylic acid. In the monomers of the quaternary ammonium type, there are other harmful impurities; and all of the above have prompted various techniques for the purification of the subject monomers [see, for example, British Patent Specification No. 1,459,811].